The present invention relates to liquid crystal light valves which are activated optically to effect switching between an optical ON state and an optical OFF state, and which are utilized for the intermediate recording medium of an optical printer, graphic display device, light shutter, image processing device, optical information processing system and so on.
There are known optically switching liquid crystal light valves of the various types which may be classified in terms of optical switching mode and liquid crystal mode. The known optical switching modes include an opto-electric conversion mode of irradiating light onto photo-conductive film, through which liquid crystal film is indirectly electrically biased, to optically change the electroconductivity of photo-conductive film to thereby electrically activate the liquid crystal film, and another opto-thermal conversion mode of scanning a laser beam along liquid crystal film to thermally change the optical state of liquid crystal film. The known optical liquid crystal modes of switching between ON and OFF states include phase transition mode of smectic A (SmA) liquid crystal, phase transition mode between nematic (Ne) liquid crystal phase and cholesteric (Ch) liquid crystal phase, dynamic scattering mode (DSM) of liquid crystal, twist nematic (TN) mode, super twist nematic (STN) mode and so on.
The opto-electro conversion mode is superior to the thermal conversion mode in view of switching speed and therefore is expected for application of displaying a moving image. However, in the known opto-electro conversion mode, the liquid crystal film maintains its activated state only during the irradiation of light thereonto. After stopping the irradiation of light, the activated or ON state of liquid crystal can be momentarily held due to the CR time constant of liquid crystal film and thereafter immediately returns to the non-activated or OFF state.
In the case of using twist nematic liquid crystal film having a normal twist angle of 90.degree. in the opto-electric conversion mode light valve, a cathode ray tube (CRT) is utilized as an irradiating light source so as to use its afterglow for maintaining the activated state. However, in such case, the resolution of the light valve is determined substantially by that of the CRT. Therefore, there are various drawbacks such as large dimension and high cost which are needed to improve the resolution of CRT.
On the other hand, there has been proposed another light irradiation source of a scanned laser beam. In this case, however, since the twist nematic liquid crystal holds its ON state only for a moment, it is difficult to improve the resolution of the light valve. In order to obtain the large scale image display of high resolution with such type of light valve, it would be necessary to repeatedly scan the laser beam along the identical path at high speed to thereby necessitate an ultra high speed laser beam scanner with ultra high resolution and ultra-high-sensitive photo-conductive film, and there would be many technical problems to be solved.